1. Field of the Invention
The present invention relates to a diversity receiver of a type which is provided with two antennas positioned with a certain space in between or two antennas having different characteristics, and which selects one antenna by a switching operation for connecting the selected antenna with the receiver, and more particularly, to a diversity receiver which is suitable for use in an FM radio receiver equipped in a motor vehicle.
2. Description of the Prior Art
A prior art diversity receiver is disclosed, for example, in Japanese Patent Laid-Open Publication No. 62-294340, in which one of the two antennae is selected in accordance with the noise level in the signal, and the selected antenna is connected to the receiver. As shown in FIG. 1, the prior art diversity receiver includes first and second antennae 1 and 2 and antenna switching circuit 3 for selecting one of the first and second antennae 1 and 2. Front end 4, IF amplifier 5, and FM detector 6 are provided for converting the FM radio signal received at the antenna to an IF (intermediate frequency) signal, amplifying, and FM detecting the IF signal, respectively. Stereo multiplexer 7 is provided for separating right and left channel stereo signals from the output signal of FM detector 6. A combination of high-pass filter 8 and noise wave detector 9 define a noise detection circuit 10 which detects the high frequency noise signal in the output signal of FM detector 6. A trigger circuit 11 is provided for producing a trigger pulse when the output signal from noise detection circuit 10 is greater than a predetermined level, and T-FF (trigger flip-flop) 12 is provided for producing a switching signal according to the trigger pulse.
A diversity receiver of this type switches the antenna when high frequency noise signal as detected by the FM detector 6 becomes greater than the predetermined level, and thereby maintaining good FM signal reception. In addition, a diversity receiver of the type shown in FIG. 1 further includes a counter 13 which counts the trigger pulses generated by trigger circuit 11, and a monostable multivibrator 14 which controls the operation of counter 13. By a control signal generated from counter 13 in accordance with the generation frequency of the trigger pulse, the stereo separation of the stereo multiplexer 7 is controlled to reduce the noise level.
Therefore, if the circuit as shown in FIG. 1 is used, good reception with no noise signal can be obtained.
However, in a diversity receiver as shown in FIG. 1, if the detection sensitivity of the noise signal is increased, erroneous operation caused by white noise during reception of weak field signals may easily occur. Furthermore, if detection sensitivity of the noise signal is lowered to prevent such an erroneous operation, the sensitivity will be insufficient for signals of a middle or strong field strength, and accurate detection of the noise signal will not be possible.